The physiology of the human eye includes an anterior chamber located between the cornea, or outer surface of the clear part of the eye, and the iris, the pigmented portion of the eye that is responsive to light, and a posterior chamber, filled with vitreous humor. A crystalline lens, which includes a lens matrix contained within a capsular bag, is located behind the iris and separates the iris from the posterior chamber. The crystalline lens is attached to the ciliary muscle by cord-like structures called zonules. Lining the rear of the posterior chamber is the retina, the light sensing organ of the eye, that is an extension of the optic nerve. In young, healthy eyes, contraction and relaxation of the ciliary muscle shapes the natural crystalline lens to the appropriate optical configuration for focusing light rays entering the eye on the retina.
As the natural crystalline lens ages, however, the structure of the lens matrix of the crystalline lens changes, becoming hazy and relatively inflexible. Eventually, the hazing of the lens matrix may progress to the point where the lens is considered cataractous, which may seriously occlude the amount of light passing through the crystalline lens and ultimately onto the retina. Fortunately, modern surgical techniques have been developed which allow removal of the cataractous lens matrix so that light may once again pass unimpeded onto the retina.
Presently, a cataractous crystalline lens matrix is removed from an eye using a procedure whereby the cataractous natural lens matrix is extracted from the capsular bag of the lens through an anterior capsulotomy. Typically, the cataractous lens matrix is removed from the capsular bag through the anterior capsulotomy using phaco-emulsification and aspiration. Alternatively, the cataractous lens matrix may be removed using several other well known techniques whereby the cataractous material is broken up and aspirated from the capsular bag. After extraction of the cataractous lens matrix, an intraocular lens may be implanted within the remaining capsular bag. However, while the procedure to remove the emulsified natural lens can be accomplished with about a three millimeter incision in the cornea, about at least a six millimeter incision is required to accommodate the full diameter of the intraocular lens to be implanted.
In order to reduce the size of the incision required for implantation of an intraocular lens, and thus limit the trauma to the eye, intraocular lenses made of relatively soft material that can be rolled, folded or otherwise deformed for insertion into the eye were developed, replacing conventional intraocular lenses made of relatively hard material, such as polymethylmethacrylate (PMMA). Soft intraocular lens must exhibit a number of important mechanical and physical properties to be suitable as an implant. For instance, soft intraocular lenses should have low glass transition temperatures so that they can be readily folded for implantation at room temperature. In addition, the thickness of the intraocular lens should be minimized in order to reduce the overall size of the folded or rolled lens. Thus, soft intraocular lenses should have a high refractive index so that the lenses will have the requisite refractory power at a minimal thickness. The lenses should also exhibit a high degree of softness to improve the foldability of the lenses, thereby reducing the size of the folded lens, while still retaining other mechanical properties, such as tensile strength and folding recoverability. Further, the lenses must be optically clear.
Prior art soft intraocular lenses made of silicone materials typically have very low glass transition temperatures (lower than −100° C.), permitting them to be readily folded or rolled at room temperature. However, other properties of silicone lenses could be improved in order to minimize the size of the folded lens. It would be desirable to provide a silicone material for use as an intraocular lens that has a high refractive index and softness to reduce the size of the folded lens. It would also be desirable to provide a silicone material that has other properties suitable for use as an intraocular lens such as high folding recoverability and optical clarity.
What has been needed and heretofore unavailable, is a silicone material having improved properties, including a high refractive index, softness, optical clarity and excellent folding recoverability, for use in intraocular lenses. The present invention satisfies these needs and others.